The present invention relates to a method or apparatus for cleaning and classifying solid particles having different sizes or weights, particularly seeds and grains.
There has always been a need to separate or classify material having solid particles of different sizes, weights, shapes or densities into various fractions.
In the processing of seeds, such as sesame seeds, it has been found that the impurity content of a crop delivered to a cleaning and processing plant is typically between 5 and 15%. The impurity content depends largely on climatic conditions during the corp. Good maintenance of the plantation and adequate storage in the farm contribute to lower content of impurities.
The composition by weight of the common impurities in a typical sesame seed harvest is as follows: seeds, 38.60%; leaves, 1.20%; stems, 50.80%; fibered particles 0.03%; earthy stones, 2.70%; grits, 0.24%; metallic particles, 0.09%; animal excrements, 2.30%; dead insects, 4.00%; and insect eggs, 0.04%.
One device used to remove impurities from seed crop is an airscreening cleaner. Such a device has a first stack of meshes arranged in parallel but inclined slightly to the horizontal. Material is deposited on the top mesh in the stack and has the largest mesh clearance, to collect the large impurities such as leaves and stems while allowing seeds and other smaller impurities to fall through the mesh. A second mesh beneath the top mesh has a medium mesh clearance and removes impurities in the same manner. A lower mesh having a relatively small mesh clearance allows small impurities to fall through, while retaining the medium-sized material for further processing. The medium-sized material is then processed through a second stack of meshes having a closer range of mesh sizes.
Using such a machine, it has been found that the vegetable content of the impurities is about 90% of global impurity, the animal impurity about 7% of global impurity and the mineral content about 3% of global impurity. The mammal and insect excrement in the unclean material is typically about 2.3% of the global impurities, which is equivalent to about 1000 mg. (1 gram) of excrement per pound of seeds for averaged global impurities of about 10%.
The U.S.D.A. and A.S.T.A. (American Spice Trading Association) maximum tolerance of mammalian excrement in sesame seeds is only 5 mg./lb. While the U.S.D.A. and A.S.T.A. set a maximum foreign matter content of 0.5%, which is equivalent to 99.5% purity, the results indicate that in order to meet the required level of 1-5 mg./lb. of excrement/seed, a theoretical purity of 99.9% appears to be required. This would equate to reducing the trash or impurity content to 1% of its an initial content, a very difficult task.
One method of removing further impurities from seeds (after much of the impurities have been removed using an airscreening cleaner as described above) is by using a gravity table. Such devices claim to have a capacity of about 3500 lb./hr. for sesame seeds, no claim being made regarding purity. By feeding relatively clean material having an initial purity rate of 99.3-99.6% into such a cleaner, a final purity rate of 99.8% may be achieved. However, this rate is achieved after recycling the material one or two times, and at a capacity of only about 500-600 lb/.hr., much less than 3500 lb./hr. Such an arrangement would require many machines working both in series and in parallel (to avoid a bottleneck in production) to achieve an acceptable purity rate at a relatively high production capacity, requiring a high investment cost. Moreover, repeated recycling of the seed product results in mechanical fatigue of the seeds and grains, increasing the quantity of broken seeds/grains in the product and thereby actually resulting in an increase in impurity matter (the broken seeds) which must be removed. Thus recycling in an attempt to increase purity is in a sense counter productive. Further, the broken seeds often increase the acidity and cause the product to deteriorate.
There is thus a need to provide a cleaner and classifier arrangement which provides a high purity content of seeds and grains at a high capacity, with a limited investment.